1. Field Of The Invention
The present invention relates to an apparatus for extruding thermoplastic material such as polyvinylchloride. More particularly, the present invention relates to apparatus for extruding a film of such plastic material in which the thickness of such film is increased along longitudinally extending zones at select zone locations across the film width.
2. Description Of The Prior Art
It is well known to the prior art that thermoplastic materials such as polyvinylchloride may be pressure extruded at viscous flow temperatures into extremely thin film sections. It is also well known to coat one or both surfaces of a paperboard substrate with such thin plastic film for the purpose of fabricating liquid cartons or containers.
Pursuant to the teachings of K. Thompson et al, U.S. Pat. No. 3,482,278 and Glenn M. Violette et al, U.S. Pat. No. 3,700,525, the strength of liquid containers is improved by applying a greater thickness of plastic film in those regions of the paperboard substrate web to be converted as the liquid container bottom.
Profiled variations in the thickness of an extruded plastic film began with the process itself; usually the unintentional result of a poorly formed or adjusted extrusion die lip. With the development of adjustable lip equipment, the lip opening along the die length (film width) was merely adjusted to a greater gap space in those areas where increased film thickness was desired. This technique allows only gradually tapered profile variations within the warp capacity of the adjustable lip element.
The patent disclosure of Thompson et al teaches an extruder apparatus design whereby the thickness profile of issued film is precisely determined by means of tapered relief zones cut into the adjustable lip or jaw of an extrusion machine.
The Violette et al apparatus accomplishes the same result by means of a threaded rod having a profiled thread root disposed symmetrically between oppositely facing die jaws within the extruded flow channel.
Although both, the Violette et al and Thompson et al profiling techniques and apparatus are more effective than warping the die lip, it will be noted that such techniques lack flexibility in terms of changes between profile patterns. In the case of the Thompson et al device, the entire adjustable lip of the die assembly must be changed. Violette et al requires that the profiling rod be exchanged for another of different profile pattern. In either case, operation of the extruder equipment must cease to permit a partial disassembly of the die. Depending on the frequency of occurrence, such operational interruptions present a major obstacle to the practice of any profiling technique.
Due to the fact that extrusion temperatures range from 200.degree. to 250.degree. F., several hours are required to cool the entire mass of the extruder assembly sufficiently to permit the required work. Moreover, each element that is removed from the assembly must be thoroughly cleaned prior to re-assembly. As much as an entire production day may be lost for the singular reason of changing a profile pattern.
It is, therefore, an object of the present invention to teach the construction of an extruder die apparatus whereby the machine may be adjusted to lay any of several profile patterns without interruption of the machine operation.
Another object of the present invention is to teach the construction of an extruder die apparatus that requires no disassembly of the die for the purpose of profile changes.